This invention relates to a supported catalyst having exceptionally small catalyst nanoparticles deposited on support particles in relatively high loading ratios, and methods of making same.
Wang et al., xe2x80x9cPreparation of Tractable Platinum, Rhodium, and Ruthenium Nanoclusters with Small Particle Size in Organic Media,xe2x80x9d Chem. Mater., v. 12, pp. 1622-1627 (2000) and Chinese Patent App. No. CN1259395A disclose xe2x80x9cunprotectedxe2x80x9d platinum nanoclusters and methods of their manufacture, i.e., platinum nanocluster colloids in organic solvents without protecting agents such as surfactants, polymers, or organic ligands.
U.S. Pat. No. 4,629,709 discloses non-noble metal microaggregates obtained by irradiating a solution of a metal salt and an oxidizing radical scavenger with ionizing radiation. The reference discloses the use of the dispersed microaggregates as catalysts for the photoreduction of water into hydrogen.
U.S. Pat. No. 4,136,059 discloses a method for producing a dispersion of platinum particles by mixing an aqueous solution of chloroplatinic acid or a salt thereof with sodium dithionate and hydrogen peroxide.
U.S. Pat. No. 5,489,563 discloses a method of making a ternary platinum alloy catalyst on a conductive carbon carrier by concurrently precipitating alloy components onto the carbon carrier by reduction of their nitrate salts.
Table I demonstrates the relationship of particle size and Pt loading in commercial catalysts available from Degussa AG, Dxc3xcsseldorf, Germany (reported in Ruth et al., xe2x80x9cElectrocatalytic Systems for PEM Fuel Cells Recent Developments at DMC2xe2x80x9d, Abstract for Fuel Cell Seminar 2000). Higher catalyst (Pt) loading appears to be linked to larger catalyst particle size. Larger particle size reduces the available surface area and therefore results in lowered catalytic activity. Table I also lists the theoretical catalyst surface area as calculated from particle size.
Table II demonstrates the relationship of particle size and Pt loading in commercial catalysts available from E-TEK Div. of De Nora N. A., Somerset, N.J. (reported at E-TEK website http://www.etek-inc.com/C1-7.html). Again, higher catalyst (Pt) loading appears to be linked to larger catalyst particle size. Table II also lists the theoretical catalyst surface area as calculated from particle size.
Briefly, the present invention provides a supported catalyst comprising catalyst metal nanoparticles having an average particle size of 3.0 nm or less, more typically 2.5 nm or less, more typically 2.0 nm or less, and most typically 1.8 nm or less, and typically having a standard deviation of particle size of 0.5 nm or less, which are supported on support particles, wherein the weight of catalyst metal nanoparticles in the supported catalyst is 30% or more of the weight of the supported catalyst, more typically 40% or more and more typically 50% or more. Typical catalyst metals are selected from platinum, palladium, ruthenium, rhodium, iridium, osmium, molybdenum, tungsten, iron, nickel and tin. Typical support particles are carbon.
In another aspect, the present invention provides a method of making a supported catalyst comprising the steps of: a) providing a solution of metal chlorides of one or more catalyst metals in solvent system containing at least one polyalcohol, typically ethylene glycol containing less than 10% water; b) forming a colloidal suspension of unprotected catalyst metal nanoparticles by raising the pH of the solution, typically to a pH of 10 or higher, and heating said solution, typically to 125xc2x0 C. or higher; c) adding support particles to the colloidal suspension; and d) depositing the unprotected catalyst metal nanoparticles on the support particles by lowering the pH of said suspension, typically to a pH of 6.5 or lower, typically by addition of nitric acid.
What has not been described in the art, and is provided by the present invention, is supported catalyst having exceptionally small catalyst nanoparticles deposited on support particles in relatively high loading ratios, and methods of making such supported catalysts by depositing unprotected catalyst nanoparticles on support particles.
In this application:
xe2x80x9cunprotected,xe2x80x9d as used in reference to colloids of metal nanoparticles in organic or aqueous solvent, means dispersed in weakly coordinating or non-coordinating solvent without protecting agents such as surfactants, polymers, or organic ligands; and
xe2x80x9cparticle sizexe2x80x9d refers to a particle""s average diameter.
It is an advantage of the present invention to provide supported catalysts that provide superior performance in electrochemical cells such as fuel cells.